Pouches can vary by material and also how well sealed from the factory.
Some have a cellophane over-wrap, which also helps.
Then, the variables of what additives (PG, etc) and how much can further alter the longevity.
The "squeeze" test will give you an idea. Give the pouch a gentle squeeze and take note of the "feel" and amount of give. Don't get all Kung-Fu grip on it, because you will turn dry tobacco into mummy dust that way.
Drier tobacco feels different in the squeeze than moist tobacco does.
I use my own "ghetto" moisture control method. If a pouch or bag starts to feel drier than I want, I dip both ends of a cotton swab in water and drop it into the pouch before closing back up. This works well for tins, also.
For a larger bag, I've used a 2x2 medical gauze square in the same manner.
If I recall, my pouches of Amphora seemed to have little or no humectants, but stayed within a desired range over a month or two.
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